Offshore seismic prospecting



March 1959 w. c. KNUDSEN OFFSHORE SEISMIC PROSPECTING Filed Aug. 27, 1956 PIC-3.3

FIG. 1

W/LL/A' C. K UDSEN INVENTOR ATTORNEYS United States OFFSHORE SEISMIC PROSPECTING Application August 27, 1956, Serial No. 606,324

9 Claims. (Cl. 181--.5)

- My invention relates to a source for use in offshore seismic exploration and particularly to such a source in which the bubble pulse is suppressed.

In offshore seismic exploration a charge of explosive is detonated below the surface of the water; the energy from the charge passes through the bottom in the form of an acoustic wave and is reflected from deep-lying formations to detectors at the surface of the water. This arrangement has proven satisfactory in places where the explosive may be placed near the surface of the water. It has been found more desirable, however, in some areas to place the'explosive near the bottom of deep water, thereby introducing additional problems related to the depth of the charge below the surface of the water.

When the charge initially explodes, it sends out a primary explosive pulse from which the useful information as to the positions of subterranean strata is derived. It has been found however, that a charge which is detonated at an appreciable depth below the surface tends to pulsate and emit secondary pulses following the primary pulse. These secondary pulses are apparently caused by the gaseous bubble which is created by detonation of the explosive charge and which expands to some diameter and then contracts, the end of each contraction giving rise to an additional energy pulse. Under some conditions there are several expansions and contractions of the gaseous bubble, resulting in several secondary pulses. These secondary pulses are objectionable in that the energy therefrom is reflected from subsurface interfaces in a manner similar to that resulting from the energy of the primary pulse, and the arrivals of this reflected secondary pulse energy tend to mask or obscure the desired events on the seismic record.

' gEiforts have been made to solve this problem by placing the explosive charge sufiiciently close to the surface to permit the initial explosive bubble to blow out at the surface before the gaseous bubble contracts, thus preventing the generation of secondary pulses. Where the explosive must be placed some distance below the surface, a pair of explosive sources have been used to cause interference with the oscillation of the gaseous bubbles and thereby decrease the strength of the secondary pulses.

In order to solve the bubble pulse problem, I have provided a single source which greatly inhibits the emission of secondary pulses. In accordance with my invention, the secondary pulses resulting from oscillations of the gas bubble generated by detonation of the charge are reduced or substantially eliminated by mechanically restricting the flow of water toward and away from the charge to thus absorb the energy in the oscillations. The mechanical restriction is accomplished through the use of a restricting member of a suitable type placed in such proximity to the explosive charge that the flow of water toward and away from the charge is restricted. Such restricting member may be provided with perforations through which the water flows from the explosion of the charge so that the energy contained in the gas bubble is dissipated in forcing water through the perforations.

'1 atent The restricting member does not attenuate the primary shock wave, but it impedes the flow of water from the source as the explosive bubble increases in size. pressure differential developed across the restricting member by the flow of water through the perforations reduces the energy present in the bubble oscillation system and thus reduces or eliminates the energy available for creation of the undesirable secondary pulses.

The perforated restricting member may completely surround the charge, in which case the member may have the form of a cylinder, a sphere, a polyhedron or two or more concentric spheres. Alternatively, the restricting member may be positioned to selectively restrict the water flow in one or more directions from the charge, in which case the member may be in the form of one or more perforated plates positioned adjacent the charge.

I have determined that the elimination of bubble pulses is, in fact, a problem of energy dissipation. Thus, the present explosive source is arranged to dissipate the energy which is associated with radial movement of the water. The use of a perforated restricting member adjacent the explosive charge introduces a sufficient obstacle to the flow of water that a pressure differential is developed across the member. Energy is dissipated in forcing water through the perforations in the member, and the energy dissipated in this manner is removed from the bubble oscillation system to prevent or greatly reduce the secondary pulses.

The member may be made of a wire mesh or it may be a metal surface having perforations. Other types of. restricting members may be used without departing from the scope of my invention if they are so selected that they constrict the flow of water away from and toward the position of the explosive charge and cause a pressure difference in the direction of flow of the water away from the source, thus causing an irreversible dissipation of energy.

The novel features of my invention are set forth. with more particularity in the accompanying claims. The invention itself, however, with respect to the details thereof, together with its additional objects and advantages, may be better understood from the following description of specific embodiments with reference to the accompanying drawings in which:

Fig. 1 shows schematically a seismic prospecting arrangement in which the explosive charge is placed in a spherical envelope;

Fig. 2 shows schematically a charge within two concentric spheres;

Fig. 3 shows an arrangement in which the charge is placed within a polyhedron; and

Fig. 4 shows the charge between two mesh screens.

As shown in Fig. l, a recording boat 5 has a string of detectors 7 at the surface of the water near the explosive charge 9. The explosive charge 9 is suspended from the surface within a perforated metal container or mesh envelope 11 which surrounds the charge 9 except for a small aperture through which the charge 9 has been inserted. In this and the subsequent embodiments, the restricting member is preferably designed, such as through the use of wire lines or the like, so that the charges may be placed therein without the necessity of bringing the member to the surface of the water, thus facilitating rapid operation.

In general, the envelope must be such that the average pressure drop across the screen as water flows from the center of the explosion is at least equal to the hydrostatic pressure at the location of the charge. The largest instantaneous pressure drop across the envelope will be approximately twice the average value of the pressure drop; I

The diameter of the envelope may be from one-half Patented Mar. 17, 1959 to one and one-half times the maximum diameter of the gaseous bubble, but preferably, the diameter of the envelope is approximately the same as the maximum diameter of the explosive bubble. The maximum diameter of the explosive bubble is approximately proportional to the cube root of the weight of the explosive. Consequently, the maximum diameter of the explosive bubble does not change rapidly with changes in the amount of the explosive charge. For this reason, a mesh envelope of ten foot diameter may be used for five to ninety pound charges of dynamite. Dynamite, black powder, TNT and other explosive materials commonly used in seismic prospecting have substantially the same energy per pound, so that the diameter of the metal envelope is substantially independent of the nature of the explosive. The limitation on the minimum radius for the envelope is determined by the bursting strength of the metal in the envelope.

The restricting screen member may be constructed of any suitable material, but I prefer to use stainless steel plate having perforations punched therein. In the spherical embodiment, the surface of the sphere is perforated with circular holes such that the ratio of metal area to total area on the surface of the sphere may be 0.7. To withstand the pressure that will be developed, the tensile strength per foot of length of the stainless steel plate used to make the sphere must be at least 50,000 pounds per foot. If holes are punched in stainless steel so that each hole is surrounded by six others all at the same distance and the ratio of hole diameter to separation between the holes is 0.57, the above strength requirement can be met with a plate 0.105 inch thick. Such a shell used in a ten foot diameter sphere around a twenty pound charge of black powder is effective to decrease greatly the tendency of the charge to emit secondary or bubble pulses.

The use of such an envelope or screen serves as an obstacle retarding the flow of water and thereby causing a pressure differential across the envelope. The energy dissipated by the screen is equal to the volume of water flowing through the screen multiplied by the change in pressure across the screen. It would appear intuitively that a device which is useful to dissipate energy and thereby prevent or attenuate the generation of secondary pulses as water flows into the screen would serve to attenuate the emission of the primary pulse. I have found, however, that the primary shock wave is emitted as a large amplitude acoustic pulse when the explosive charge detonates and the explosive bubble begins to expand. The energy of the primary shock wave is affected only slightly by the screen since it is associated with a phenomenon unrelated to volume flow of water through the mesh. The perforated envelope withdraws energy from the oscillating system on expansion and contraction of the explosive bubble. The secondary pulses, which are emitted as the bubble ends its contraction, are decreased in magnitude because the explosive bubble does not contract to as small a diameter, part of the energy of the oscillation having been dissipated by the envelope. Thus, although the primary and secondary shock waves are both emitted at the same place, the primary shock wave is emitted before energy is dissipated by the mesh.

In view of the great amount of energy that must be dissipated, and the high pressure resulting from the detonation of the explosive charge, it is desirable in some instances to dissipate energy by a two-stage process. Fig. 2 shows a device by which pressure drops are created across two envelopes, the pressure drops being such that the energy dissipation by the two envelopes is sufficient to prevent objectionable bubble oscillation. The charge 9 of explosive is enclosed in an inner envelope 11 and an outer envelope 13. When the charge 9 has twenty pounds of dynamite, the diameter of the inner envelope 11 may be six feet and the diameter of the outer envelope 13 is seven feet. The two envelopes cause two pressure drops as water flows away from and toward the center of the mesh envelopes 11 and 13, thus providing a two-step dissipation of the energy stored in the gas bubble.

As an alternate embodiment, a regular polyhedron may be used of any shape in which all of the faces of the envelope are at substantially the same distance from the center of the envelope. Fig. 3 shows an example of one polyhedron which would be useful in the practice of my invention. The nearest face of the polyhedron is located at a distance from the center of the polyhedron approximately equal to the maximum radius of the explosive bubble. This polyhedron is a cube in which the corners have been replaced by a triangular surface having apexes lying at the centers of edges of the cube. A cube may also be used.

As an additional alternative, the envelope need not completely enclose the explosive source. An unenclosed restricting member may be utilized which is of sufficient size and shape and spaced properly from the explosive charge to cause a substantial portion of the water propelled from the area immediately surrounding the explosive bubble to pass through the perforations and hence to dissipate a substantial portion of the energy of the flowing water. For example, a single perforated metal plate may be anchored to one side of the explosive charge. If the plate is unanchored, it will tend to move with the water and thus dissipate less energy so that anchored plates are preferably utilized. Fig. 4 shows the use of two perforated plates 15 and 17 on either side of the explosive charge 9. The plates 15 and 17 are held together by rods or cables 18 and are supported from the member 19 which supports the charge 9. While I have shown the use of two plane plates, the members 15 and 17 may be hemispherical or polygonal. If the two opposed surfaces are joined together, neither will move appreciably as a result of the explosion.

An additional embodiment of an unenclosed restricting member may be in the form of a cylinder formed of perforated metal and having open ends, the charge being disposed inside the cylinder and the longitudinal axis of the cylinder being disposed in a suitable direction. Such a cylinder is readily fabricated by rolling a perforated metal plate into the desired shape, and, preferably, adding suitable reinforcing members in the central portion of the cylinder. I have found that such a cylinder having a length of ten feet and a diameter of five feet, with reinforcing members, performs very satisfactorily.

While I have shown and described specific embodiments of my invention, they are used by way of example only and I do not intend to limit my invention except as set forth in the appended claims.

I claim:

1. Apparatus for decreasing secondary pulsations after a primary explosive pulse in water comprising, in combination, a charge of explosive substantially enclosed in a perforated envelope, said envelope having surfaces distant from said charge by a distance approximately equal to the maximum radius of the gaseous cavity in the water due to the primary explosive pulse.

2. Apparatus for decreasing secondary pulsations after a primary explosive pulse in water comprising, in combination, a charge of explosive, a mesh container substantially surrounding said charge of explosive and separated from said charge of explosive by a distance approximately equal to the maximum radius of the gaseous cavity in the water due to the primary explosive pulse.

3. Apparatus for suppressing the emission of secondary pulses from an explosive charge under water comprising a perforated spherical metal envelope having an explosive charge at its center and having a diameter from one-half to one and one-half times the maximum diameter of the gaseous bubble which would have been formed around the explosive source in the absence of the envelope.

4. A seismic source comprising a charge of explosive having approximately the explosive energy of a twenty pound charge of black powder, a perforated metal envelope concentric with the charge and having a radius of approximately five feet, the ratio of metal area to total area of the envelope being 0.7.

5. Apparatus for decreasing secondary pulsations following a primary explosive pulse in water comprising, in combination, a charge of explosive, a plurality of perforated metal envelopes substantially concentric with said charge of explosive and substantially surrounding said charge of explosive, said envelopes being adapted to retard the flow of water through said envelopes.

6. Apparatus for decreasing secondary pulsations after a primary explosive pulse in water comprising, in combi nation, a charge of explosives substantially enclosed in a perforated envelope, said envelope being in the shape of a regular polyhedron and having all faces of said envelope distant from said charge by a distance approximately equal to the maximum radius of the gaseous cavity in the water due to the primary explosive pulse.

7. Apparatus for decreasing secondary pulsations after a primary explosive pulse in Water comprising, in combination, a charge of explosive substantially enclosed in a perforated spherical envelope, said envelope having sur faces distant from said charge by a distance approximately equal to the maximum radius of the gaseous cavity in the Water due to the primary explosive pulse.

8. The method for preventing secondary pulses in a body of water following a primary pulse resulting from the explosion of a submerged explosive charge comprising the steps of submerging an explosive charge in the water, substantially surrounding said explosive charge with perforated plate means through which the water can flow when propelled by said explosive charge, spacing said plate means from said charge a distance approximately equal to the maximum radius of the gaseous cavity formed in the Water due to the primary explosive pulse, and exploding said explosive charge.

9. The method for preventing secondary pulses in a body of water following a primary pulse resulting from the explosion of a submerged explosive charge comprising subrnerging an explosive charge in the water, positioning on opposite sides of said explosive charge perforated plate means through which the water can flow when propelled by said explosive charge and at a distance from said explosive charge substantially equal to the radius of the gaseous cavity formed in the water due to the primary explosive pulse, and exploding said explosive charge.

References Cited in the file of this patent UNITED STATES PATENTS 816,251 Pet-mecky Mar. 27, 1906 1,313,582 Cowan Aug. 19, 1919 1,764,039 Cooper June 17, 1930 2,159,578 Weinman May 23, 1939 2,382,380 Buttner Aug. 14, 1945 2,414,349 Alexander Jan. 14, 1947 2,604,954 Mann July 29, 1952 2,609,885 Silverman Sept. 9, 1952 2,629,044 Marfisi Feb. 17, 1953 

